The invention relates to a process for preparing insoluble and safe pigment dyes by first preparing a colloidal dispersion of a vat dye, preferably from the group comprising the anthraquinone dyes, the indigoid dyes and the phthalocyanine dyes, in an aqueous alkaline medium by agitation to form a solution of the leucobase of the dye. Under conditions which rule out premature oxidation, a finely divided substrate is introduced into this solution. An oxidizing agent is then added to the solution to oxidize the leucobase to form the dye, which thereby precipitates and deposits on the substrate. After the oxidation, a strong acid in excess is added to the alkaline medium. The dye-coated substrate is then separated from the medium, washed and dried.
British Pat. No. 1,000,531 already discloses a process of this kind, wherein vat dyes are reduced in alkali solution to form the corresponding leuco compound. By mixing the solution with an aqueous dispersion of a pigment and thereafter oxidizing the leuco compound, the dye is deposited on the pigment. The pigment used therein is a sodium aluminum silicate. The dye-coated pigment thus obtained is used for industrial purposes, in particular in thermoplastics, rubber, printing inks, lacquers and paints.
German Pat. No. 2,044,104 discloses a similar process, where the pigment or substrate to be coated used is titanium dioxide and where the combination of the high hiding power of the titanium dioxide with the color stability of vat dyes is emphasized as particularly advantageous. Since the pigment dye is not only insoluble but also safe, it can indeed be ingested with foodstuffs or medicaments or be brought into contact with the human body by means of cosmetics. Therefore, the pigment dye is suitable for use as a coloring additive in foodstuffs, medicaments, cosmetics, perfumeries, toys or personal accessories.
In particular, in the latter mentioned areas of use it is an absolute requirement that the pigment dye used be free of harmful impurities. Examples of impurities which are not tolerable for these uses are the heavy metals barium, lead and arsenic and also organic impurities such as, for example, aromatic amides, polycyclic aromatic hydrocarbons and intermediates from the synthesis of the dyes.
It was therefore customary in the field to start from very pure dyes in order to obtain safe, pure pigment dyes.
However, this has the disadvantage that these grades of dye are very costly, evidently as a result of expensive purifications. Although the dye is applied to the substrate to be coated only as a coating, this nonetheless constitutes a considerable cost factor which pushes up the cost of the completed pigment dye.
There thus exists a need for a process whereby these pigment dyes can be prepared in a less costly manner without impairing the quality, in particular the purity and safeness, of the pigments.